Nitrogen oxide present in exhaust or emissions have been removed by, for example, a method in which the nitrogen oxide is further oxidized and then absorbed in an alkali or a method comprising reducing it to nitrogen by using ammonia, hydrogen, carbon monoxide, hydrocarbons, etc. as a reducing agent.
These conventional methods have their own disadvantages. That is, the former method requires a means for handling the alkaline waste liquid to prevent environmental pollution. Of the latter method, the method of using ammonia as a reducing agent involves the problem that ammonia reacts with sulfur oxides in the emissions to form salts, resulting in a reduction in catalyst activity. When in using hydrogen, carbon monoxide, or a hydrocarbon as a reducing agent, the reducing agent preferentially undergoes reaction with oxygen because oxygen is present in a higher concentration than nitrogen oxide in emission. This means that substantial reduction of nitrogen oxide requires a large quantity of the reducing agent.
It has recently been proposed to catalytically decomposing nitrogen oxide in the absence of a reducing agent. However, known catalysts for direct decomposition of nitrogen oxide have not yet been put to practical use due to their low decomposing activity.
On the other hand, H type zeolite, Cu ion-exchanged ZSM-5, etc. have also been proposed as a catalyst for catalytic reduction of nitrogen oxide using a hydrocarbon or an oxygen-containing organic compound as a reducing agent. In particular, H type ZSM-5 (SiO.sub.2 /Al.sub.2 O.sub.3 molar ratio=30 to 40) are regarded optimal. However, even the H type ZSM-5 is not deemed to have sufficient reducing activity. In particular, when the emissions have some water content, the zeolite structure is dealuminated, showing a sudden drop in its performance. It has thus been demanded to develop a catalyst for catalytic reduction of nitrogen oxide which exhibits higher reduction activity and shows a high durability even if the emissions have some water content.